By means of fumigating one-year-old seedlings in open top chambers, this paper studied the impact of automobile exhaust on the pH value, relative conductivity, malondialdehyde (MDA) and chlorophyll contents, superoxide dismutase (SOD) and peroxidase (POD) activities, and ascorbic acid (ASA) content in the seedlings foliage of four tree species, Acer mono, Malus baccata, Prunus ussuriensis, and Acer ginnala. During the fumigation, the seedlings were exposed to the same exhaust gas concentration (25 microg x m(-3), indicated by the NO2 concentration in exhaust) for different durations (1, 3, 5, 7 d), and to different concentrations (40, 60, 80, 100 microg NO2 x m(-3)) for same duration (2 h). The results showed that the pH value and the chlorophyll and ascorbic acid (ASA) contents decreased, whereas the relative conductivity, malondialdehyde (MDA) content, and superoxide dismutase (SOD) and peroxidase (POD) activities increased with increasing fumigation duration and exhaust concentration. Obvious interspecies variations in term of physiological features were found. After treated 7 days with 25 microg NO2 x m(-3) and treated 2 h with 100 microg NO2 x m(-3), only a 1.5% and 2.7% decrease of cell juice pH was found in A. ginnala, respectively, compared to the control. The corresponding data for P. ussuriensis was 9.42% and 13.89%, followed by M. baccata. The chlorophyll content of A. mono, A. ginnala, M. baccata and P. ussuriensis was 83.0%, 71.3%, 68.7% and 54.9%, respectively of the control after 7 days treated with 25 microg NO2 x m(-3), and the corresponding data under 100 microg NO2 x m(-3) treatment was 60.2%, 73.1%, 43.4% and 51.2%, respectively. The decrease of ASA content and Acer ginnala was less in A. mono than in M. baccata and P. ussuriensis. The relative conductivity and MDA content of A. mono increased respectively by 68.1% and 52.5% in compared with control, while those of A. ginnala had the least increment. As for the 100 microg NO2 x m(-3) treatment, the maximum increases of relative conductivity (99.8%) and MDA content (52.5%) were found in M. baccata, while the least were found in A. mono and A. ginnala. The SOD and POD activities generally increased under the highest concentration of exhaust, with the exception of M. baccata. To summarize, almost all the test physiological features of A. ginnala were least affected, suggesting its great tolerance to exhaust stress, but in contrary, the greatest variation of the physiological features and the dramatic decline of SOD and POD activities of M. baccata at the highest exhaust concentration indicated that this tree species had the least tolerance to exhaust stress.